#pragma GCC optimize("Ofast,unroll-loops")
//#pragma GCC optimize("conserve-stack")
#include<bits/stdc++.h>
//#define int long long
#define ll long long
#define down cout<<'\n';
#define debug cout<<" cucuucucuuu",down
#define NHP ios_base::sync_with_stdio(0);cout.tie(0);cin.tie(0);
#define modwwe int t;cin>>t; while(t--)
#define bit(i,j) (i>>j&1)
#define sobit(a) __builtin_popcountll(a)
#define task "test"
#define fin(x) freopen(x".inp","r",stdin)
#define fou(x) freopen(x".out","w",stdout)
#define pb push_back
#define checktime cerr << (double)clock() / CLOCKS_PER_SEC * 1000 << " ms";
using namespace std;
#define getchar_unlocked getchar
inline int scan()
{
char c = getchar_unlocked();
int x = 0;
while (c < '0' || c > '9')
{
c = getchar_unlocked();
}
while (c >= '0' && c <= '9')
{
x = (x << 1) + (x << 3) + c - '0';
c = getchar_unlocked();
}
return x;
}
void phongbeo();
const int inf = 1e9;
const int mod2 = 1e9 + 7;
const int mod1 = 998244353;
struct icd
{
long double a;
int b;
};
struct ib
{
int a;
int b;
};
struct ic
{
int a, b, c;
};
struct id
{
int a, b, c, d;
};
struct ie
{
int a, b, c, d, e;
};
int n, m, s1, s2, s4, s3, sf, k, s5, s6, mx, s7, s8, s9, mx2, res, dem2 = 0, dem = 0, s33, dem3, dem4, l, r, mid, l2, r2, center;
int i, s10, s12,k1,k2,k3,s11,t,lim,w;
int kk;
int el = 19;
main()
{
if(fopen(task".inp","r"))
{
fin(task);
fou(task);
}
NHP
/// cin>>s1;
///modwwe
phongbeo();
// checktime
}
string s;
int dp[2][401][401][3];///dp(0,1) i j f1 f2
/// -> i la cot G
/// ->j la cot Y
int a[401][3];
int gf[3];
int p1[3];
int p2[3];
void phongbeo()
{
cin>>n;
cin>>s;
s=" "+s;
p1[0]=0;
p1[1]=1;
p1[2]=0;
p2[0]=0;
p2[1]=0;
p2[2]=1;
dp[0][0][0][0]=dp[0][0][0][1]=dp[0][0][0][2]=0;
for(int i=1; i<=n; i++)
{
if(s[i]=='R') s2=0;
else if(s[i]=='G')s2=1;
else s2=2;
a[i][s2]++;
for(int f=0; f<=2; f++)
a[i][f]+=a[i-1][f];
}
for(int i=1; i<=n; i++)
{
for(int j=0; j<=(i); j++)
for(int z=0; z+j<=i; z++)
for(int f=0; f<=2; f++)
dp[i&1][j][z][f]=inf;
for(int j=0; j<=i-1; j++)
for(int z=0; z+j<=i-1; z++)
for(int f=0; f<=2; f++)
for(int f2=0; f2<=2; f2++)
{
if(f==f2) continue;
s3=a[i-1][0]-(i-1-j-z);
s4=a[i-1][1]-j;
s5=a[i-1][2]-z;
s6=max(0,s3)+max(0,s4)+max(0,s5);
dp[i&1][j+p1[f2]][z+p2[f2]][f2]=min( dp[i&1][j+p1[f2]][z+p2[f2]][f2],dp[(i-1)&1][j][z][f]+s6);
}
}
if(min({dp[n&1][a[n][1]][a[n][2]][0],dp[n&1][a[n][1]][a[n][2]][1],dp[n&1][a[n][1]][a[n][2]][2]})==inf) cout<<-1;
else cout<<min({dp[n&1][a[n][1]][a[n][2]][0],dp[n&1][a[n][1]][a[n][2]][1],dp[n&1][a[n][1]][a[n][2]][2]});
}
